Although the HIV/AIDS pandemic is principally due to infection by HIV-1, a different retrovirus has emerged as another cause of AIDS. This so-called “HIV-2” virus was first isolated from AIDS patients in West Africa in 1986, and was subsequently detected as an infectious agent for the first time in the United States the following year. Fewer than 100 cases of HIV-2 had been reported in the United States through the end of 1994. Despite this seemingly low number, HIV-2 is being identified as the etiologic agent in growing numbers of immunosuppressive diseases that are clinically indistinguishable from AIDS cases that result from HIV-1 infection (Kanki et al., Science 232:238 (1986); Kanki et al., Science 236:827 (1987); Clavel et al., Science 233:343 (1986); Clavel et al., N. Engl. J. Med. 316:1180 (1987)). Although HIV-2 is related to HIV-1 by its morphology and tropism for CD4+ cells, it clearly is a distinct virus and not merely an envelope variant of HIV-1.
Indeed, since HIV-2 is only distantly related to HIV-1, with approximately 50% amino acid conservation in the gag and pol proteins and less than 30% conservation in the env gene products, its presence is not effectively detected by serologic assays used for detecting HIV-1 infection (Constantine N T, AIDS 7:1 (1993); Markovitz D M, Ann. Intern. Med. 118:211 (1993)). As a result, attempts have been made to develop nucleic acid probes that can be used for specifically detecting HIV-2 viral nucleic acids.
Interestingly, the genomes of both HIV-1 and HIV-2 show substantial sequence heterogeneity among different isolates. As a consequence of this heterogeneity, it has been impossible to find substantial regions of absolute sequence conservation between all isolates of HIV-1 or all isolates of HIV-2 (see published European Patent Application EP 0 887 427). Indeed, numerous viral isolates with unique polynucleotide sequences have been identified for each of these viruses, a factor that further complicates the construction of probes for reliable and effective nucleic acid testing.
Since, like HIV-1, HIV-2 also is transmissible through exchange of body fluids, including blood and plasma, it is important to be able to detect infected body fluids before antibodies to the virus are detectable or symptoms are evident in an infected individual. For protection of patients who might otherwise receive an HIV-2-infected body fluid (e.g., whole blood or plasma during transfusion), or products derived from donated blood or plasma, it is particularly important to detect the presence of the virus in the donated body fluid to prevent its use in such procedures or products. It is also important that procedures and reagents used for detecting HIV-2 can detect relatively low numbers of viral copies which may be present in an infected individual, who may be a donor, during the early stages of infection.
Assays and reagents for detecting HIV-2 have been previously disclosed in, for example, U.S. Pat. Nos. 6,020,123, 5,688,637, 5,545,726 and 5,310,651; European Patent Nos. EP 0404625 B1 and EP 0239425 B1; and published European Patent Application Nos. EP 1026236 A2, EP 0887427 A2.